1. Field of the Invention
The solid electrolyte electrochemical cell of this invention provides concurrent gas phase electrocatalytic oxidative dimerization of methane at one side of the solid electrolyte and reduction of carbon dioxide to gaseous hydrocarbon products at the opposite side of the solid electrolyte. The electrochemical cell may use a solid electrolyte of an oxygen vacancy conducting type or a proton transferring type capable of transferring any proton mediating ion. The process of this invention uses a solid electrolyte electrochemical cell to produce C.sub.2 hydrocarbon species on each side of the electrolyte concurrently by gas phase reaction, using methane reactant on the anode side and carbon dioxide reactant on the cathode side.
2. Description of the Prior Art
The complete electrochemical oxidation of methane to CO.sub.2 and H.sub.2 O in the anode compartment of a solid oxide fuel cell, after its initial steam reformation to hydrogen, has been used in the conversion of natural gas into DC electricity, Handbook of Batteries and Fuel Cells, Ed. David Linden, 43-26 to 43-33, published by McGraw-Hill Book Company (1984).
The chemical synthesis of ethylene by oxidative coupling of methane using Sn, Pb, Sb, Bi, Tl, Cd, and Mn oxide catalysts is taught by Keller, G. E., and Bhasin, M. M., "Synthesis of Ethylene via Oxidative Coupling of Methane," Journal of Catalvsis, 73, 9-19 (1982). However, the Keller, et al. article teaches Li, Mg, Zn, Ti, Zr, Mo, Fe, Cr, W, Cu, Ag, Pt, Ce, V, B, and Al oxides to have little or no such catalytic activity. The chemical synthesis of ethylene directly from methane in the presence of oxygen over LiCladded transition metal oxide catalysts providing high selectivity and yield is taught by Otsuka, K., Liu, Q., Hatano, M. and Morikawa, A., "Synthesis of Ethylene by Partial Oxidation of Methane over the Oxides of Transition Elements with LiCl", Chemistrv Letters, The Chemical Society of Japan, 903-906 (1986). Chemical partial oxidation of methane over LiCl-Sm.sub.2 O.sub.3 catalyst to C.sub.2 products, ethylene and ethane, with a high ethylene selectivity is taught by Otsuka, K., Liu, Q., and Morikawa, A., "Selective Synthesis of Ethylene by Partial Oxidation of Methane over LiCl-Sm.sub.2 O.sub.3 " J. Chem. Soc., Chem. Commun., 586-587 (1986). Chemical conversion of methane to ethane and ethylene under oxygen limiting conditions over La.sub.2 O.sub.3 is taught by Lin, C., Campbell, K. D., Wang, J., and Lunsford, J. H., "Oxidative Dimerization of Methane over Lanthanum Oxide," J. Phys. Chem., 90, 534-537 (1986).
Oxidative coupling of methane over Ag and Bi.sub.2 O.sub.3 -Ag catalysts was carried out with oxygen electrochemically pumped through yttria-stabilized zirconia and it was found that the oxygen pumped to the Bi.sub.2 O.sub.3 -Ag catalyst showed higher catalytic activity and selectivity for the production of C.sub.2 compounds compared to surface oxygen from the gas phase, Otsuka, K., Yokoyama, S., and Morikawa, A., "Catalytic Activity - and Selectivity - Control for Oxidative Coupling of Methane by Oxygen-Pumping through Yttria-Stabilized Zirconia," Chemistry Letters, The Chemical of Japan, 319-322 (1985). Electrochemical driving of O.sup.2 species through solid electrolyte yttria-stabilized zirconia decreased selectivity to C.sub.2 hydrocarbons and decreases the rate of production of C.sub.2 H.sub.4 using an Ag-Li/MgO catalyst electrode, Seimanides, S. and Stoukides, M., "Electrochemical Modification of Ag-MgO Catalyst Electrodes during Methane Oxidation," J. Electrochem. Soc., 1535-1536, July, 1986. Rare earth metal oxides Sm.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Er.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Y.sub.2 O.sub.3, and Bi.sub.2 O.sub.3 have been shown to have good catalytic activity and selectivity in chemical oxidative coupling of methane, Sm.sub.2 O.sub.3 being the most active and selective catalyst in the formation of C.sub.2 compounds, Otsuka, K., Jinno, K., and Morikawa, A., "The Catalysts Active and Selective in Oxidative Coupling of Methane," Chemistry Letters, The Chemical Society of Japan, 499-500 (1985).
Indirect reduction of CO.sub.2 on a mercury electrode in an aqueous electrolyte, pH 7, containing TiCl.sub.3, Na.sub.2 MoO.sub.4 and pyrocatechol where the total Faradaic efficiency for cathodic hydrocarbon generation was about 0.2 percent at 7mA/cm.sup.2, with methane being the major hydrocarbon component, is taught by Petrova, G. N. and Efimova, O. N., Elektrokhimiva, 19(7), 978 (1983). CO.sub.2 has been shown to be reducible to CH.sub.4, CO, and methanol at ruthenium cathodes in CO.sub.2 saturated aqueous Na.sub.2 /SO.sub.4 electrolyte with Faradaic efficiencies for CH.sub.4 production up to 42 percent at current densities up 0.11mA/cm.sup.2 by Frese, Jr., K. W. and Leach, S., "Electrochemical Reduction of Carbon Dioxide to Methane, Methanol, and CO on Ru Electrodes," J. Electrochem. Soc.. 132, 259 (1985).
Copper, 99.99 percent pure, was used as a cathode with 0.5 M KHCO.sub.3 electrolyte for the electrochemical reduction of CO.sub.2 at ambient temperature and current density of 5.0 mA/cm.sup.2 for 30 to 60 minutes with Faradaic efficiencies for CH.sub.4 of 37 to 40 percent, Hori, Y., Kikuchi, K., and Suzuki, S., "Production of CO and CH.sub.4 in Electrochemical Reduction of CO.sub.2 at Metal Electrodes in Aqueous Hydrogencarbonate Solution," Chem. Lett., 1695 (1985). In later work high purity copper cathodes, 99.999 percent, were used for the electrochemical reduction of CO.sub.2 in 0.5M KHCO.sub.3 electrolyte in a cell operated at a current of 5mA/cm.sup.2 for 30 minutes at temperatures of 0.degree. C. and 40.degree. C. shows Faradaic efficiency for production of CH.sub.4 drops from 60 percent at 0.degree. to 5 percent at 40,.degree.; C.sub.2 H.sub.4 increases from 3 percent at 0.degree. to 18 percent at 40.degree.; while hydrogen production increases from 20 percent at 0.degree. to 45 percent at 40.degree.. It is stated that 99.99 percent pure copper cut the Faradaic efficiencies to about one-third of those obtained with 99.999 percent pure copper, Hori, Y., Kikuchi, K., Murata, A., and Suzuki, S., "Production of Methane and Ethylene in Electrochemical Reduction of Carbon Dioxide at Copper Electrode in Aqueous Hydrogencarbonate Solution," Chem. Lett., 897 (1986). Later work of electrochemical reduction of CO.sub.2 at a 99.999 percent pure copper cathode in aqueous electrolytes of KCl, KClO.sub.4, and K.sub.2 SO.sub.4 at 19.degree. C. and current density of 5mA/cm.sup.2 showed Faradaic yields of C.sub.2 H.sub.4 of as high as in the order of 48 percent, CH.sub.4 12 percent and EtOH 21 percent, Hori, Y., Murata, A., Takahashi, R., and Suzuki, S., "Enhanced Formation of Ethylene and Alcohols at Ambient Temperature and Pressure Electrochemical Reduction of Carbon Dioxide at a Copper Electrode," J. Chem. Soc., Chem. Commun, 17, 1988.
Electroreduction of CO at a 99.999 percent pure copper cathode in an aqueous catholyte of KHCO.sub.3 at ambient temperature for 30 minutes showed hydrogen the predominant product and at 1.0mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 22 percent, CH.sub.4 1 percent; 2.5mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 21 percent, CH.sub.4 16 percent and at 5.0mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 16 percent, CH.sub.4 6 percent, Hori, Y., Murata, A., Takahashi, R., and Suzuki, S., "Electroreduction of CO to CH.sub.4 and C.sub.2 H.sub.4 at a Copper Electrode in Aqueous Solutions at Ambient Temperature and Pressure," J. Am. Chem. Soc., 109. 5022 (1987). Similar work by the same authors showed electroreduction of CO at a 99.999 percent pure copper cathode in an aqueous 0.1 M KHCO.sub.3 pH 9.6 catholyte at 19.degree. C. at 2.5mA/cm.sup.2 resulted in Faradaic production C.sub.2 H.sub.4 of 21.2 percent; CH.sub.4 of 16.3 percent; EtOH of 10.9 percent; and 45.5 percent H.sub.2, Hori, Y., Murata, A., Takahashi, R., and Suzuki, S., "Electrochemical Reduction of Carbon Monoxide to Hydrocarbons at Various Metal Electrodes in Aqueous Solution," Chem. Lett., 1665 (1987 ).
In the reduction of CO.sub.2 to CH.sub.4 using 99.9 percent pure cold rolled B 370 copper cathodes with a CO.sub.2 saturated 0.5M KHCO.sub.3 electrolyte, Faradaic efficiencies of 33 percent were achieved for CH.sub.4 at current densities up to 38 mA/cm.sup.2 with no C.sub.2 H.sub.4 being detected, Cook, R. L., McDuff, R. C., and Sammells, A. F., "Electrochemical Reduction of Carbon Dioxide to Methane at High Current Densities," J. Electrochem. Soc.. 134, 1873 (1987).
Electrochemical reduction of CO.sub.2 to CH.sub.4 and C.sub.2 H.sub.4 was shown to occur at copper/Nafion electrodes (solid polymer electrolyte structures) at Faradaic efficiencies of about 9 percent for each CH.sub.4 at E=-200V vs. SCE using 2mM H.sub.2 SO.sub.4 counter solution at a temperature of 22.degree. C., Dewulf, D. W. and Bard, A. J., "The Electrochemical Reduction of CO.sub.2 to CH.sub.4 and C.sub.2 H.sub.4 at Cu/NAFION Electrodes (Solid Polymer Electrolyte Structures)," Cat. Lett. 1, 73-80, (1988).